Most digital cameras don't actually take a picture the instant you hit the button. What they actually do to capture an image is scan over the frame either vertically or horizontally. So basically, not all parts of an image are recorded at exactly same time (the top right could be a little ahead of the bottom left, etc.). Hence it being called "rolling shutter".

Filmed and edited on the iPhone 4... make sure to go full screen and to go to the vimeo site to be in HD... quite impressive for a phone !